What are trans fats? Trans fats are created when vegetable oils have been altered by adding hydrogen to unsaturated fatty acids, which increases the number of hard-to-reach double bonds. These double bonds make them more difficult for the body to break down, which causes them to build up in our bodies. The best way to avoid dietary trans fat is by switching from partially hydrogenated oils (like margarine), frying in lard or tallow, or using canola oil instead of corn oil and vegetable shortening.